QByteArray sum

[high_flyer]

I am sorry, I simply can't follow.
I got that you are doing some kind of a check sum, fair enough.
But everything else I just cant understand.
The code you posted wont compile and I can't understand it.
It *seems* to me you are very sure about your math, and yet, it appears to me that you still have a problem with it, but what that problem is I just can't make out of your post, or what the actual problem or question is.
Maybe someone else here understands more and can help.
Sorry.

EDIT:
maybe if you post a bit more of your code, the real code, where you calculate your checksum, and where you use, it will be clearer.